Timber frame extension ??

We'd like to build a small extension under permitted development on a brick and block cavity wall Bungalow on a thick concrete raft.

We hope to clad the whole building with weatherboard so was hoping I could DIY the extension in timber. I'm no builder but a good diy'er with good wood skills as I used to repair wooden boats. I'm just not versed in the details of building construction.

The extension would be about 2.7 by 4,.5 metres wide with a pitch roof.

Any advice - dont do it, get on with it etc. I guess I need to speak to building control to see what floor they will accept, i.e. More concrete slab (its very thick) or foundations and joists.

Its not a very warm bungalow despite the cavity being filled, we may put some thin insulation in when cladding (which apparently I do need permission for).

How do i match and hopefully improve the thermal in the timber frame and avoid cold bridging through the timber frame please?

Thanks again Kiab, we were also looking at a garden room in Sips. Its quite surprising though how the price jumps up from a basic finish yourself Sips box to a supplied completed room (windows, cladding, roofing, elecs etc).

What are my options for house extension foundation please. We currently have a very thick concrete raft, for a timber frame extension would it be likely I would need such substantial foundations? I know it depends on the local building control but I wonder if trench foundation would be OK.